Solution and process for increasing the solderability and corrosion resistance of a metal or metal alloy surface

ABSTRACT

Described is a new a solution comprising a phosphorous compound and optionally a solderability-enhancing compound and its use in a process for increasing the solderability and corrosion resistance of a metal or metal alloy surface.

FIELD OF THE DISCLOSURE

The invention relates to a solution comprising a phosphorous compoundand optionally a solderability-enhancing compound and its use in aprocess for increasing the solderability and corrosion resistance of ametal or metal alloy surface.

BACKGROUND OF THE INVENTION

Metal coatings, particularly of tin or its alloys are frequentlydeposited onto surfaces of copper- and nickel-based workpieces in orderto prevent the copper or nickel-based surface from oxidizing ortarnishing and to enhance solderability.

Under conditions such as elevated temperatures in air or in otheroxidizing atmospheres, for example tin or other metal-coated surfaces ofelectronic lead frames and electrical connectors have a tendency to formoxide films during periods of shipment and storage between manufactureand assembly into electronic devices. The oxide coats discolor thesurface of the tin-coated surface and impart a yellowish color whichmany consumers consider unacceptable. Furthermore, the oxide may degradethe contact resistance of a coated electrical terminal. A tarnish-freesurface has lower electrical contact resistance and better solderabilitythan an oxide coated surface.

Tin-based layers are also used in lead finishes for integrated circuit(“IC”) fabrication. A thin layer of tin or tin alloy is applied as thefinal step for passive components such as capacitors and transistors.

There are many factors that determine good solderability, the three mostimportant of which are extent of surface oxide formation (corrosion),amount of co-deposited carbon, and extent of intermetallic compoundformation. Surface oxide formation is a natural occurring processbecause it is thermodynamically favorable. The rate of formation of thesurface oxide depends on the temperature and time. The higher thetemperature and the longer the exposure time, the thicker the surfaceoxide formed. In electroplated tin or tin alloy coatings or deposits,surface oxide formation also depends on the surface morphology of thecoating or deposit. When comparing pure tin to tin alloy coatings, forexample, tin alloys generally form less or thinner surface oxides whenall other conditions are equal.

Generally, it is the aim to produce a tarnish-free surface, which haslower electrical contact resistance and better solderability than anoxide coated surface.

Fuchs et al., in U.S. Pat. No. 5,853,797 disclose a method and solutionfor providing corrosion protection of coated electrical contact surfaceswhich involves exposure of such surfaces to a solution containingphosphonates, lubricants and various volatile organic solvents.Evaporation of such solvents for disposal is fraught with environmentalconcerns such as handling, hazard to workers, and disposal of waste intostreams.

Fan et al., in U.S. Pat. Appl. No. 2005/0268991 A1 disclose a method forenhancing corrosion resistance of a tin-based surface on a workpieceinvolving contacting the tin-based surface with a composition comprisinga phosphonic acid compound and water to form a phosphorus-based filmover the tin-based coating thereby inhibiting corrosion of the tin-basedsurface. Phosphonic acid containing compositions having a concentrationup to about 30 vol. % of an organic solvent, and water. However, the useof organic solvents is disadvantageous since they are volatile underprocess conditions and often hazardous.

Lau et al., in U.S. Pat. Appl. No. 2006/0237097 A1 disclose a method forinhibiting corrosion of metals and metal alloys by treating them withcompositions containing inorganic and organic phosphoric acids likealkylphosphoric acids that prevent oxide formation on the metals andmetal alloys.

EP 0 009 247 B relates to the use of alkyl monophosphonic acidcontaining from 8 to 10 carbon atoms in admixture with surfactants ascorrosion inhibitors against metals in acid solutions. The metals areselected from the group consisting of aluminum and aluminum alloys,chromium-nickel-steel, ordinary steel, brass and copper.

EP 1 221 497 A2 relates to a method for inhibiting the formation ofstains, especially water stains, on the exterior surfaces of aluminumalloy products. The method entails contacting the exterior surfaces ofthese products, particularly sheet or plate products, extrusions and/orforgings made from 5000 to 6000

Series aluminum alloys, with an organophosphonic or organophosphinicacid-derived material. Preferably, liquid forms of this material areadded to an alcohol or water-based carrier solution, then sprayed,dipped, painted or rolled onto the surfaces of flat sheet or plateproducts to enhance their brightness.

GB 2 331 942 A relates to a composition comprising at least one organophosphonate or organo phosphonate species used for the treatment of avariety of metal surfaces to inhibit corrosion and improve adherence ofcoatings. The composition can include a homopolymer or copolymer of anorgano phosphonate or organo phosphonate species such as vinylphosphonicacid, vinylidene-1,1-diphosphonic acid or phenyl vinyl phosphonic acid.

U.S. Pat. No. 3,630,790 describes a method of protecting metal surfacesfrom corrosion which comprises contacting the metal with an organophosphonic, phosphonous or phosphinic acid.

Although the above mentioned methods are all directed to treatment ofmetal and metal alloy surfaces in order to preserve their qualityregarding solderability and appearance, there is still a need for anenvironmentally friendly process offering improved corrosion resistanceas well as whisker suppression.

SUMMARY OF THE DISCLOSURE

The present invention relates to an aqueous solution which comprises aphosphorous compound and, optionally a solderability-enhancing compoundas defined herein below.

Further, the present invention relates to a process for increasing thesolderability and corrosion resistance of a metal or metal alloysurfaces wherein the surface is contacted with this aqueous solution.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a treatment sequence of plating and applying thecomposition according to the present invention.

FIG. 2 shows the water dissemination on an untreated tin-plated samplewithout any post-treatment derived from comparative example 1.

FIG. 3 shows the water repelling effect of a tin-plated sample treatedwith a composition according to example 2.

FIG. 4 shows the water repelling effect of a tin-plated sample treatedwith a composition according to example 3.

FIG. 5 shows the water repelling effect of a tin-plated sample treatedwith a composition according to example 4.

DETAILED DESCRIPTION OF THE INVENTION

The invention is directed to a solution and a process for enhancingcorrosion resistance of metal or metal alloy surfaces e.g. of nickel,copper, silver, gold, palladium, platinum, rhodium, ruthenium, indiumand alloys thereof. Preferably the metal or metal alloy surface is a tinor tin alloy surface. Examples of tin alloy surfaces comprise SnPb,SnCu, SnBi, SnAg and SnAgCu.

For purposes of illustration, one such workpiece is an electroniccomponent such as an electronic lead frame, a passive component, a bumpon a wafer or is an electrical connector. Other suitable metal surfacescomprise zinc, aluminum, iron or copper or their alloy based surfaces.

The solution according to the invention is applicable to any metalsurface, preferably a tin or tin alloy surface, whether part of anelectronic device, engineering, functional, decorative, or otherwise.With regard to tin-based surfaces for electronic devices, the methodenhances corrosion resistance and also preserves solderability of a tinor tin alloy surface during storage prior to a soldering operationinvolving reflow of a portion of the tin-based surface.

The solution of the present invention is also applicable for very thinmetal surfaces, e.g. gold, which possess pores. Such pores can be sealedwith the solutions of the present invention and prevent the oxideformation of underlying layers of e.g. nickel, copper or tin under thegold layer.

In accordance with the invention, the metal-based surface is immersed orotherwise contacted with a composition comprising a phosphorouscompound, a solderability-enhancing compound and water to form aphosphorus-based film over the tin-based surface. This film inhibitscorrosion of the tin-based surface, increases wettability andsolderability and surprisingly also prevents whisker formation.

The aqueous solution for treating a metal surface according to thepresent invention comprises

-   -   (a) at least one phosphorous compound or its salt represented by        the followings formulas

-   -   wherein R1 is selected from the group comprising H, OH,        C₁-C₂₀-alkyl, substituted or unsubstituted, linear or branched,        C₁-C₆-alkaryl, linear or branched, substituted or unsubstituted        and aryl, substituted or unsubstituted,    -   R2 and R3 are the same or different and are selected        independently from the group consisting of H, a suitable counter        ion like sodium or potassium, C₁-C₂₀-alkyl, substituted or        unsubstituted, linear or branched, C₁-C₆-alkaryl, linear or        branched, substituted or unsubstituted and aryl, substituted or        unsubstituted, C₁-C₂₀-alkylene, substituted or unsubstituted,        linear or branched and wherein n is an integer ranging from 1 to        10,    -   preferably, R1 of the phosphorous compounds according to        formula I. is selected from the group consisting of H, OH,        methyl, ethyl, n-propyl, isopropyl, n-hexyl, isohexyl, n-heptyl,        isoheptyl, n-octyl, isooctyl, n-nonyl, isononyl, n-decyl,        isodecyl, n-undecyl, isodecyl, n-dodecyl, isododecyl,        preferably, R2 and R3 of the phosphorous compounds III. are        selected independently from the group consisting of H or a        suitable counter ion like sodium or potassium, methyl, ethyl,        n-propyl, isopropyl, n-hexyl, isohexyl, n-heptyl, isoheptyl,        n-octyl, isooctyl, n-nonyl, isononyl, n-decyl, isodecyl,        n-undecyl, isodecyl, n-dodecyl, isododecyl,    -   R4 is F(CF₂CF₂)_(z), z ranges from 1 to 7, x is 1 or 2, y is 1        or 2 and x+y is 3, and optionally    -   (b) at least one solderability-enhancing compound or its salt        represented by the following formula

wherein m, n, o and p are integers ranging from 0 to 200 and are thesame or different and m+n+o+p is at least 2. Preferably m+n+o+p rangesfrom 4 to to 100, more preferably from 10 to 50 and wherein R1 and R7are the same or different and are selected independently from the groupconsisting of H, a suitable counter ion like sodium or potassium,C₁-C₂₀-alkyl, substituted or unsubstituted, linear or branched,C₁-C₆-alkaryl, linear or branched, allyl, aryl, sulfate, phosphate,halide and sulfonate and wherein each of the R2, R3, R5 and R6 groupsmay be the same or different and are selected independently from thegroup consisting of H, C₁-C₆-alkyl, linear or branched, substituted orunsubstituted and wherein R4 is selected from the group consisting ofC₁-C₁₂-alkylene, linear or branched, substituted or unsubstituted,arylene 1,2-, 1,3- and 1,4-substituted, naphthylene, 1,3-, 1,4- 1,5-1,6- and 1,8-substituted, higher annulated arylene, cycloalkylene,—O—(CH₂(CH₂)_(n)OR1, wherein R1 has the meaning defined above, andmoieties represented by the following formulas

wherein the substitution independently is 1,2-, 1,3- or 1,4 for eachring and wherein q and r are the same or different and rangeindependently from 0 to 10 and R8 and R9 are selected independently fromthe group consisting of H and C₁-C₆-alkyl, linear or branched.

Substituted alkyl, alkaryl and aryl groups described herein arehydrocarbyl moieties which are substituted with at least one atom otherthan carbon and hydrogen, including moieties in which a carbon chainatom is substituted with a hetero atom such as nitrogen, oxygen,silicon, phosphorous, boron, sulfur, or a halogen atom. The hydrocarbylmoieties may be substituted with one or more of the followingsubstituents: halogen, heterocyclo, alkoxy, alkenoxy, alkynoxy, aryloxy,hydroxy, protected hydroxy, hydroxycarbonyl, keto, acyl, acyloxy, nitro,amino, amido, nitro, phosphono, cyano, thiol, ketals, acetals, estersand ethers.

Preferred are aqueous solutions, wherein R1 and R7 of thesolderability-enhancing compound III. are selected independently fromthe group consisting of H, methyl, sodium, potassium, halide, sulfate,phosphate and sulfonate.

Preferred are aqueous solutions, wherein R2, R3, R5 and R6 of thesolderability-enhancing compound III. are selected independently fromthe group consisting of H, methyl, ethyl, n-propyl and isopropyl.

Preferred are aqueous solutions wherein R4 of thesolderability-enhancing compound III. is selected from the grouprepresented by the following formulas

and wherein R8 and R9 are selected from the group consisting of H,methyl, ethyl, n-propyl and isopropyl.

Solderability-enhancing compounds according to formula III. having thefollowing formulas are particularly preferred.

and wherein n=1-20, preferably 3-8.

and wherein n=1-20, preferably 2-10.

wherein n=1-20, preferably 2-7.

The aqueous compositions according to the present invention usually havea pH of 1-8, preferably of 2-5. In order to ensure a constant pH valueduring operation preferably a buffer system is applied to the solution.Suitable buffer systems comprise formic acid/formiate, tartaricacid/tartrate, citric acid/citrate, acetic acid/acetate and oxalicacid/oxalate. Preferably, the sodium or potassium salt of theaforementioned acid salts are used. Besides the mentioned acids andcorresponding salts, all buffer systems can be applied which result in apH value of the aqueous compositions of 1-8, preferably of 2-5.

The buffer concentration is in the range of 5-200 g/l for the acid andof 1-200 g/l for its corresponding salt.

The at least one phosphor compound a) represented by the formulas I. andII. of the aqueous solutions is preferably used in an amount of 0.0001to 0.05 mol/l, more preferably 0.001 to 0.01 mol/l.

The optional at least one solderability-enhancing compound (b)represented by the formula III. is generally used in an amount of 0.0001to 0.1 mol/l, preferably 0.001 to 0.005 mol/l.

Optionally, the solution may additionally contain at least one watersoluble organic solvent selected from the group comprising C₂-C₆alcohols, C₂-C₆ glycols and esters thereof. The concentration of saidorganic solvent ranges from 0.001 ml/l to 100 ml/l, more preferred from0.1 ml/l to 20 ml.

Optionally, the solution may additionally contain an anti-foaming agentwhich is commercially available.

The aqueous compositions according to the present invention are usuallyapplied during a process sequence as shown in FIG. 1.

Plating preferably with tin or a tin alloy is by standard methods knownin the art and for example described in Metal Finishing, Guidebook &Directory, 2006, p. 266-277. Generally, metal plating can be both byelectrolytic as well as electroless plating methods.

Tin or tin alloy plating is preferred, but also zinc, aluminium, iron orcopper plating may be employed.

For example, a typical tin plating bath may include one or more watersoluble tin salts such as tin sulfate, tin alkyl sulfonates such as tinsulfonate, tin alkanol sulfonic acid, and tin halides such as tinchloride and tin fluoroborate. The bath also includes electrolytes suchas sulfuric acid, alkyl sulfonates, alkanol sulfonates and halide saltsto provide an electro-conductive matrix. Surfactants also may beincluded as well as other conventional additives to provide a desirabletin layer. The amounts of the components are conventional and are wellknown in the art and may be obtained from the literature.

The numbers in the following paragraphs refer to the treatment sequenceas shown in FIG. 1.

The plating step 1. often is in an acidic plating bath followed by arinse step 2. with demineralised water and an optional neutralisationstep 3. applying an alkaline solution in case the plating bath is acidicand vice versa. Optionally, the plated surface is again rinsed withdemineralised water and then treated in step 5. with a solutionaccording to the present invention.

Preferably, the substrate is immersed into the solution. Alternatively,spraying or dipping is also possible.

The treatment time can vary between 1 s and 10 minutes, preferably thecontact time is at least 5 s and does not exceed 60 s. Generally, thesolutions are at temperatures of 15° C. to 60° C., preferably from 20°C. to 40° C.

The metals and metal alloys may then be optionally rinsed with water toremove any excess oxide inhibiting composition.

A post bake step may follow the treatment and rinse step by heating thesubstrate to a temperature between 100 and 200° C. and a time of 30 and120 minutes. This step is particularly preferred if the substrate is alead frame. The post baking further reduces the whisker formation.

A reflow process may follow the treatment and rinse step. Any suitablereflow process may be used. Reflow may be by heating by vapour phasereflow, laser reflow, plasma, oven melting, and passing an electricalcurrent through the metals and metal alloys, or by any other method ofheating the metals and metal alloys above their melting temperature.

The present invention is further illustrated by the following examples.

General: The substrate to be plated is a copper sheet.

The copper surface is plated with tin using a commercially available tinplating bath (Stannopure® HSM-HT, Atotech Deutschland GmbH) consistingof tin methanesulfonic acid (70 g/l tin), 200 g/1 methanesulfonic acid,wetting agent and grain refiner. The bath temperature is 40° C., thecurrent density 10 A/dm², the plating time 2.0 min and the coatthickness (Sn) 10 μm.

Example 1 Comparative

A copper sheet as described before is plated with tin according to abovementioned Stannopure® HSM-HT process.

After plating the copper sheet is treated according to the processsequence as shown in FIG. 1 and described above, except that step 5(treatment with a solution according to the present invention) isomitted.

The alkaline dip (step 4) is carried out in a solution containing 10 g/lpotassium phosphate, K₃PO₄, at room temperature for 15 s. Post baking isat 150° C. for 1 hour.

The dissemination of water on the tin-plated substrate surface having acontact angle of 35° against water is shown in FIG. 2.

Example 2

A copper sheet as described before is plated with tin according to abovementioned Stannopure® HSM-HT process.

The alkaline dip (step 4, FIG. 1) is carried out in a solutioncontaining 10 g/l potassium phosphate, K₃PO₄, at room temperature for 15s. The inventive composition according to step 5 of FIG. 1 is an aqueoussolution containing 1 g/l of a compound according to formula I. whereinn=2, R1=methyl, R2=a cis-9-octadecenyl residue and R3=H.

Post baking is at 150° C. for 1 hour.

The water repellent effect of a solution according to example 2 is shownin FIG. 3. The contact angle of water on a tin-plated substrate treatedwith a solution according to example 2 without application of step 3(FIG. 1) is 113° and when applying step 3 the contact angle againstwater is 111°.

Example 3

A copper sheet as described before is plated with tin according to abovementioned Stannopure® HSM-HT process.

The alkaline dip (step 4, FIG. 1) is carried out in a solutioncontaining 10 g/l potassium phosphate, K₃PO₄, at room temperature for 15s. The inventive composition according to step 5 of FIG. 1 is an aqueoussolution containing 1 g/l of a 1 g/l of a compound according to formulaI. wherein n=2, R1=methyl, R2=a cis-9-octadecenyl residue and R3=H, 10ml/l isopropylglycol and 10 g/lα,α′,α″-1,2,3,-Propanetriyltris[ω-hydroxypoly(oxy-1,2-ethandiyl)], CASNo. 31694-55-0

Post baking is at 150° C. for 1 hour.

The water repellent effect of a solution according to example 3 is shownin FIG. 4. The contact angle of water on a tin-plated substrate treatedwith a solution according to example 3 without application of step 3(FIG. 1) is 119° and when applying step 3 the contact angle againstwater is 114°.

Example 4

A copper sheet as described before is plated with tin according to abovementioned Stannopure® HSM-HT process.

The alkaline dip (step 4, FIG. 1) is carried out in a solutioncontaining 10 g/l potassium phosphate, K₃PO₄, at room temperature for 15s. The inventive composition according to step 5 of FIG. 1 is an aqueoussolution containing 1 g/l of a compound according to formula II. (Zonyl®UR, DuPont), 10 ml/l isopropylglycol and 10 g/lα,α′,α″-1,2,3,-Propanetriyltris[o-hydroxypoly(oxy-1,2-ethandiyl)], CASNo. 31694-55-0

Post baking is at 150° C. for 1 hour.

The water repellent effect of a solution according to example 4 is shownin FIG. 5. The contact angle of water on a tin-plated substrate treatedwith a solution according to example 4 without application of step 3(FIG. 1) is 136° and when applying step 3 the contact angle againstwater is 142°.

From the results the effect on solderability by the treatment withcompositions of the present invention becomes apparent. Compared to theprior art it also becomes apparent that the compounds can be used inconcentrations much lower than described. WO 2005/121405 A1 according toExample 1 disclosed the use of 10 g/l n-octylphosphonic acid instead of1 g/l in case of the present invention.

Additionally, the discoloration of samples prepared according toExamples 1-4 was observed. After 4.000 hours at 55° C. and 85% humidity,the tin-plated copper sheet treated according to Example 1 show yellowdiscoloration, while the tin-plated copper sheets of Examples 2-4 retainthe silver colour of the freshly plated tin surface.

FIG. 3 shows the water repellent effect of a solution according toExample 2 on a lead frame compared to an untreated sample (Example 1,FIG. 2). The contact angle theta of water on freshly plated matt tindeposits, non treated and treated with the composition according to thepresent invention had been determined by prop Shape Analysis (DSA) witha Krüss DSA10 Mk2. The contact angle for the tin-plated copper sheetstreated according to examples 2 to 4 (FIGS. 3 to 5) of larger than 110°indicate excellent water repellent properties. In contrast, thetin-plated copper sheet of Example 1 (FIG. 2) shows a not desired goodwettability (the contact angle against water is 35°).

The invention claimed is:
 1. An aqueous solution having a pH in therange of 1 to 5 for treating a tin or tin alloy surface comprising (a)at least one phosphorous compound or its salt represented by thefollowing formulas I or II:

wherein R1 is selected from the group consisting of H, OH, methyl,ethyl, n-propyl, isopropyl, n-hexyl, isohexyl, n-heptyl, isoheptyl,n-octly, isooctyl, n-nonyl, isononyl, n-decyl, isodecyl, n-undecyl,isodecyl, n-dodecyl, isododecyl, R2 and R3 are the same or different andare selected independently from the group consisting of H, a counterion, C₁-C₂₀-alkyl, substituted or unsubstituted, linear or branched,C₁-C₆-alkaryl, linear or branched, substituted or unsubstituted andaryl, substituted or unsubstituted and wherein n is an integer rangingfrom 2 to 10, R4 is F(CF₂CF₂)_(z), z ranges from 1 to 7, x is 1 or 2, yis 1 or 2 and x+y is 3 and (b) at least one solderability-enhancingcompound or its salt represented by the following formula

wherein m, n, o and p are integers ranging from 0 to 200 and are thesame or different and m+n+o+p is at least 2 wherein R1 and R7 are thesame or different and are selected independently from the groupconsisting of H, a counter ion, C₁-C₂₀-alkyl, substituted orunsubstituted, linear or branched, C₁-C₆-alkaryl, linear or branched,allyl, aryl, sulfate, phosphate, halide and sulfonate and wherein eachof the R2, R3, R5 and R6 groups may be the same or different and areselected independently from the group consisting of H and C₁-C₆-alkyl,linear or branched, substituted or unsubstituted and wherein R4 isselected from the group consisting of C₁-C₁₂-alkylene, linear orbranched, substituted or unsubstituted, arylene 1,2-, 1,3- and1,4-substituted, naphthylene, 1,3-, 1,4- 1,5- 1,6- and 1,8-substituted,higher annulated arylene, cylcloalkylene, —O—(CH₂(CH₂)_(n)OR1, whereinR1 has the meaning defined above, and moieties represented by thefollowing formula

wherein the substitution independently is 1,2-, 1,3- or 1,4 for eachring and wherein q and r are the same or different and rangeindependently from 0 to 10 and R8 and R9 are selected independently fromthe group consisting of H and C₁-C₆-alkyl, linear or branched.
 2. Anaqueous solution according to claim 1, wherein R2 and R3 of thephosphorous compound are selected independently from the groupconsisting of H, a counter ion, methyl, ethyl, n-propyl, isopropyl,n-hexyl, isohexyl, n-heptyl, isoheptyl, n-octyl, isooctyl, n-nonyl,isononyl, n-decyl, isodecyl, n-undecyl, isodecyl, n-dodecyl, andisododecyl.
 3. An aqueous solution according to claim 1, wherein R1 andR7 of the solderability-enhancing compound III are selectedindependently from the group consisting of H, methyl, sodium, potassium,halide, sulfate, phosphate and sulfonate.
 4. An aqueous solutionaccording to claim 1, wherein R2, R3, R5 and R6 of thesolderability-enhancing compound III are selected independently from thegroup consisting of H, methyl, ethyl, n-propyl and isopropyl.
 5. Anaqueous solution according to claim 1, wherein R4 of thesolderability-enhancing compound III is selected from the followingformulas

and wherein R8 and R9 are selected from the group consisting of H,methyl, ethyl, n-propyl and isopropyl.
 6. An aqueous solution accordingto claim 1, wherein the solderability-enhancing compound III is selectedfrom the group consisting of the following formulas

wherein n=1-20,

wherein n=1-20 and

wherein n=1-20.
 7. An aqueous solution according to claim 1, wherein theat least one phosphorous compound (a) represented by the formulas I-IIis used in an amount of 0.0001 to 0.05 mol/l.
 8. An aqueous solutionaccording to claim 1, wherein the at least one solderability-enhancingcompound (b) represented by the formula III. is used in an amount of0.0001 to 0.1 mol/l.
 9. An aqueous solution according to claim 1,wherein the pH value is between 2 and
 5. 10. An aqueous solutionaccording to claim 1, wherein a buffer system selected from the groupconsisting of formic acid/formate, tartaric acid/tartrate, citricacid/citrate, acetic acid/acetate and oxalic acid /oxalate is contained.11. An aqueous solution according to claim 1, wherein R1 and R7 of thesolderability-enhancing compound III are selected independently from thegroup consisting of H, methyl, sodium, potassium, halide, sulfate,phosphate and sulfonate; and R2, R3, R5 and R6 of thesolderability-enhancing compound III are selected independently from thegroup consisting of H, methyl, ethyl, n-propyl and isopropyl.
 12. Anaqueous solution according to claim 1, wherein R1 and R7 of thesolderability-enhancing compound III are selected independently from thegroup consisting of H, methyl, sodium, potassium, halide, sulfate,phosphate and sulfonate; R2, R3, R5 and R6 of thesolderability-enhancing compound III are selected independently from thegroup consisting of H, methyl, ethyl, n-propyl and isopropyl; and R4 ofthe solderability-enhancing compound III is selected from the followingformulas

and wherein R8 and R9 are selected from the group consisting of H,methyl, ethyl, n-propyl and isopropyl.
 13. A process for increasing thesolderability and corrosion resistance of a substrate having a tin ortin alloy surface wherein the surface is treated with an aqueoussolution according to claim
 1. 14. The process according to claim 13,wherein the tin alloy surface is selected from the group consisting ofSnPb, SnCu, SnBi, SnAg and SnAgCu surfaces.
 15. The process according toclaim 13 further comprising reflowing the tin or tin alloy surface ofthe substrate prior or after treatment with the aqueous solution. 16.Substrate having a tin or tin alloy surface treated with the aqueoussolution according to claim
 1. 17. Substrate according to claim 16,wherein the substrate is a tin plated semi-finished good.
 18. Substrateaccording to claim 17, wherein the substrate is a tin plate or tin wireor a lead frame, a connector or a printed circuit board.